It's important to note that FEBE is not e10s compliant as it does not run correctly in a multi-process environment. The browser(s) should already know this and set the browser.tabs.remote.autostart preference (in about:config) to false.

Make sure this preference is set properly before attempting to open FEBE options.

I'm wondering how productive it is to try to debug on an unstable OS. From the FreeBSD site:

FreeBSD-CURRENT is the “bleeding edge” of FreeBSD development and FreeBSD-CURRENT users are expected to have a high degree of technical skill. Less technical users who wish to track a development branch should track FreeBSD-STABLE instead.

FreeBSD-CURRENT is the very latest source code for FreeBSD and includes works in progress, experimental changes, and transitional mechanisms that might or might not be present in the next official release. While many FreeBSD developers compile the FreeBSD-CURRENT source code daily, there are short periods of time when the source may not be buildable. These problems are resolved as quickly as possible, but whether or not FreeBSD-CURRENT brings disaster or new functionality can be a matter of when the source code was synced.

To your knowledge, does FEBE work correctly on stable FreeBSD releases?

– you'll find the URL of your preferred repo and if you browse there, you'll find only one packaged version of firefox; of firefox-esr; of waterfox; and so on. If firefox there is already at Quantum, you'll not find 56.x packaged unless you get lucky with a different repo – however mixing repos is rarely recommended (and for what we're doing here, overkill).

Here, I locked things down weeks ago, whilst firefox was still at 56.x in a preferred repo;

GhostBSD11.1-mate-amd64.iso from http://www.ghostbsd.org/download can run a live session in VirtualBox, and includes Firefox 56.0.2. Whilst a live session is good enough to reproduce the crashing, I have no idea whether it'll be good enough to debug.

(Beyond a live session: I had less success with an installation of GhostBSD in VirtualBox – no GUI, networking not working, and so on.)

As of now, I can verify that FEBE v8.9.3.1 works with both Firefox v56.0.2 and Waterfox v56.0.4 under Windows 10. And FEBE v10.2 works with Pale Moon v27.7.2 (Windows 10).

At one time, FEBE was working fine with Ubuntu but I've haven't tested it recently. I know it does not work with Waterfox under Linux Mint 18.3. I've had limited success with OS X but don't remember with which versions and/or browsers.

There are simply too many nuances with different browsers running under different OSes to test all combinations. And I don't want to have to maintain several different versions of FEBE for each combination.

FEBE for Fx will probably not have any more updates. I will, however, try to maintain the Pale Moon version as well as a version for Pale Moon's Basilisk. But the focus will be on the Windows versions.